Electrostatic precipitators are employed for the removal of particles in various gases, particularly from smoke emanating from coal fired boilers employed in the generation of electrical power. Typically, a precipitator includes a series of electrical precipitating sections arranged to serially intercept smoke, each stage employing a plurality of parallel, vertically positioned, metal plates and between each two there is positioned spaced strands of wire lying in a plane halfway between and parallel with the plates. The wire is typically no greater in diameter than 1/8", and, in some instances, barbs, like barb wire, are attached to the wire. The wires are negatively biased with respect to the plates, which are grounded, there being a potential difference between wires and plates in the kilovolt range such that current densities are produced on the order of 10 to 50 nanoamperes per square cm (centimeter). When properly so biased, corona discharge occurs from the wires into adjacent spaces containing particles, causing them to be negatively charged. The particles are then drawn by the electrical field to the positively poled plates. Then, periodically, the plates are mechanically rapped, and the particles fall off and are collected.
One problem that is ever present with precipitators of the class described is that of the possible occurrence of what is termed back corona discharge, or corona discharged from dust layers accumulated on the plates. When this occurs, positively charged particles are produced, and they thus tend to flow away from the plates and neutralize the effect of normal negatively charged particle flow to the plates. This degrades the performance of the precipitator.
Back corona discharges arise when the normal ion current from the discharge wire element increases beyond a selected level, normally referred to as a maximum permissible current density level. Practically, this means that the bias between the wire discharge element and plates, and thus field strength must be restricted to a value which will not produce a current density in excess of the maximum permissible current density. Significantly, this maximum permissible current density level is inversely proportional to the resistivity of the particles to be precipitated out.
In the past, where coal fired boilers typically burned high sulfur coal, resistivities were on the order of 10.sup.9 to 5.times.10.sup.10 ohm cm, and in such case, current densities on the order of 20 to 60 nanoamperes per square cm (obtained by field strengths on the order of 1.5 to 3.5 kilovolts per cm) were permissible without back corona discharge and good results were obtainable. In recent years, however, because of the increased emphasis on protection of the environment, there has been, and there is now occurring, a substantial shift to the employment of low sulfur coal, and low sulfur coal typically produces particles having an increased resistivity, typically in the range of 5.times.10.sup.10 to 5.times.10.sup.12 ohm cm. This in turn creates the problem suggested, namely, that in order to operate with existing systems (and without back corona discharge) it is necessary to reduce field strengths (by reduction of applied bias) to reduce current densities. In fact, it has been found with existing systems that to avoid back corona, current densities must be reduced to typically less than 10 nanoamperes per square cm. This calls for rather severe decreases in field strength. This in turn necessarily decreases the charge imparted to particles, and the combination of reduced field strength and reduced particle charge decreases the velocity of movement of particles, and in general the efficiency of particle collection. Thus, less collection is effected for a given cross section of precipitator.
It is the object of this invention to solve the problem described and to accomplish it by a system in which the actual field strength is increased rather than decreased, obviously a desired state as indicated above.